1. Field of the Invention
This invention relates to radiation curable organosiloxane compositions. More particularly, this invention relates to one part organosiloxane compositions that cure upon exposure to ultraviolet radiation to yield gels that will not harden at low temperatures and provide suitable coatings and encapsulants for electronic components.
2. Description of the Prior Art
Organosiloxane gels are characterized by their softness and resiliency. The hardness of cured gels is typically too low to be measured on any of the Shore durometer scales, including the "00" scale. The hardness of these gels is sometimes expressed as the depth to which a probe of specified dimensions and weight will penetrate a cured gel.
One of the more popular uses of organosiloxane gels is as protective coatings and encapsulants for electrical and electronic components, particularly solid state devices such as integrated circuits, and electrical connectors. The gel prevents penetration of atmospheric moisture and other materials that can corrode or otherwise damage the component.
Two types of gels that are particularly suitable for this application are the dielectric gels described in U.S. Pat. No. 3,020,260, which issued to Nelson on Feb. 6, 1962 and in U.S. Pat. No. 4.374,967. which issued to Brown et al. on Feb. 22, 1983. The gels described in the Brown et al. patent are particularly desirable because they retain their properties, particularly softness, at very low temperatures.
The curable compositions described in the Brown et al. patent cure by a platinum-catalyzed hydrosilylation reaction between a vinyl-substituted polymethylsiloxane of specified composition and an organohydrogensiloxane containing an average of more than one silicon-bonded hydrogen atom per molecule. This curing mechanism is more desireable than others described in the prior art for the curing of organosiloxane compositions based on the relatively rapid cure rate and the absence of by-products that are typically associated with moisture curable organosiloxane compositions.
A disadvantage of compositions such as those described by Brown et al. is their relatively poor storage stability, which requires that the organohydrogensiloxane and the platinum catalyst be packaged in separate containers if the composition is to be stored for any considerable time prior to use. When all of the ingredients of the composition are blended together they require from several minutes to several hours to cure, depending upon the temperature. Another disadvantage of this type of composition is the susceptibility of the platinum-containing catalyst to deactivation by various impurities, including sulfur compounds.
The prior art describes several types of organosiloxane compositions that cure in one minute or less when exposed to ultraviolet radiation in the presence of a suitable photoinitiator.
U.S. Pat. No. 4,052,529. which issued to G. Bokerman et al. on Oct. 4, 1977 discloses compositions suitable for use as coatings for release papers. The compositions comprise (1) a polydiorganosiloxane wherein a portion of the repeating siloxane units contain mercaptoalkyl groups as substituents, (2) a cyclic methylvinylsiloxane and (3) benzophenone as the photoinitiator. The compositions contain from 1 to 5 mercapto groups per vinyl radical.
The ultraviolet light-curable compositions described in U.S. Pat. No. 4,780,486, which issued to C. Lee and M. Lutz on Oct. 25, 1988 comprise an alkenyl-substituted polydiorganosiloxane, a mercaptoalkyl-substituted polydiorganosiloxane or a polyfunctional organic mercaptan as the crosslinking agent and a photoinitiator that decomposes to yield free radicals in the presence of ultraviolet radiation. The free radicals, in turn, initiate curing of the organosiloxane composition. The molar ratio of mercaptoalkyl radicals or mercapto groups to vinyl radicals in the compositions described in this patent is from 0.67 to 2.
A disadvantage of organosiloxane compositions that cure by the radiation-initiated reaction of mercapto groups with alkenyl groups is the adverse effect the cured materials can have on the electrical performance of electrical and electronic devices that are coated with these materials due to the tendency of sulfur to corrode metals. This limits the types of substrates that can be coated with the elastomeric compositions described in the aforementioned Lee and Lutz patent, which specifies that the compositions contain from 0.5 to 1.5 alkenyl radicals per mercapto group.
U.S. Pat. No. 3,873,499, which issued to K. Michael, A. Mink and D. Mitchell on Mar. 25, 1975 describes mercaptoalkyl-substituted organosiloxane resins that cure in the presence of cyclic methylvinyl siloxanes and ultraviolet light. The resins contain mercaptopropylsiloxy, propylsiloxy, and dimethylsiloxane units. The curable compositions contain from 0.8 to 1.2 vinyl radicals per mercapto group.
U.S. Pat. No. 4,303,484, which issued to M. Takamizawa et al. on Dec. 1, 1981 discloses photocurable organosiloxane compositions suitable for use as release coatings. The compositions contain an organopolysiloxane with at least two mercaptoalkyl groups per molecule, an organopolysiloxane having at least two alkenyl groups per molecule and an organopolysiloxane containing at least one benzoin group per molecule. The molar ratio of alkenyl radicals to mercapto groups is preferably from 0.5 to 2.
The present inventors discovered that when curable compositions containing an alkenyl-substituted polyorganosiloxane of the type described in the aforementioned Brown et al. patent, a photoinitiator and a relatively small amount of a mercapto-substituted polyorganosiloxane as the curing agent are exposed to ultraviolet radiation the compositions cure in a minute or less. Moreover, cured gels prepared using the present compositions remain soft and flexible even at low temperatures and are suitable for encapsulation of hybrid circuits and other electronic devices. When applied to metal surfaces preferred compositions cause less corrosion relative to prior art compositions that cure by a similar mechanism.